(1) Field of the Invention
Besides nematic and cholesteric liquid crystals, optically active, tilted, smectic (ferroelectric) liquid crystals have also recently been used in commercial display devices.
(2) Description of Related Art
Clark and Lagerwall have been able to show that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in opto-electrical switching or display elements which have response times which are faster by a factor of up to 1000 compared with conventional TN (xe2x80x9ctwisted nematicxe2x80x9d) cells (see, for example, EP-A 0 032 362). Owing to this and other favorable properties, for example the possibility of bistable switching and the virtually viewing angle-independent contrast, FLCs are basically highly suitable for areas of application such as computer displays.
For a more detailed explanation of the technical requirements of FLCs, reference is made to European Patent Application 0 916 714 and DE-A 197 48 432.
Liquid-crystalline 3,5-diphenylisoxazoles have been described in JP 60-054375, Liq. Cryst. 1997, 22, 3746, and Liq. Cryst. 1997, 22, 265-273. The synthesis and mesogenic properties of symmetrically substituted bisalkoxy-3,5-diphenylisoxazoles has been described in Liq. Cryst. 1992, 11, 887-897, Liq. Cryst. 1992, 11, 899, Mol. Cryst. Liq. Cryst. 1992, 220, 67-75, and Mol. Cryst. Liq. Cryst. 1993, 225, 175-182. JP-A 10-333113 describes 3,5-diphenylisoxazoles for use in ferroelectric liquid-crystal mixtures.
However, since the development, in particular of ferroelectric liquid-crystal mixtures, can in no way be regarded as complete, display manufacturers are interested in a wide variety of components for mixtures, partly because only the interaction of the liquid-crystalline mixtures with the individual components of the display device or of the cells (for example the alignment layer) allows conclusions to be drawn on the quality of the liquid-crystalline mixtures too.
It has now been found that isoxazole derivatives of the formula (I) below, in which the isoxazole moiety is linked in position 3 and/or 5 to a thiophene or furan moiety, even when admixed in small amounts, have a favorable effect on the properties of liquid-crystal mixtures, in particular chiral smectic mixtures, for example regarding the dielectric anisotropy and/or the melting point, but also regarding the switching behavior, the tilt angle value and the temperature dependence of the tilt angle.
The invention therefore provides isoxazole derivatives of the formula (I) 
where the symbols and indices have the following meanings:
T is 
X is S or O
R1 and R2 are identical or different and are each hydrogen, F, CN or a straight-chain or branched C1-C20-alkyl or C2-C20-alkenyl radical (with or without asymmetric carbon atoms), where
a) one to three nonterminal CH2 groups may be replaced, independently of one another, by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94C(xe2x95x90O)xe2x80x94, with the proviso that heteroatoms cannot be linked directly to one another, and/or
b) one CH2 group may be replaced by xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94, and/or
c) one CH2 group may be replaced by xe2x80x94Si(CH3)2-, cyclopropane-1,2-diyl, oxirane-2,3-diyl, cyclobutane-1,3-diyl, cyclopentane-1,4-diyl, bicyclo[1.1.1]pentane-1,3-diyl or cyclohexane-1,4-diyl, and/or
d) one or more H atoms may be replaced, independently of one another, by F or CN,
e) in the case of an alkyl or alkenyl radical containing asymmetric carbon atoms, the asymmetric carbon atoms have xe2x80x94CH3, xe2x80x94OCH3, xe2x80x94CF3, xe2x80x94F, xe2x80x94CN and/or xe2x80x94Cl as substituents,
with the proviso that only one of R1 and R2 can be hydrogen, F or CN;
A1 and A2 are undirected and identical or different and are each phenylene-1,4-diyl, unsubstituted or monosubstituted or disubstituted by CN or F, phenylene-1,3-diyl, unsubstituted or monosubstituted or disubstituted by CN or F, cyclohexane-1,4-diyl, in which one or two H atoms may be replaced, independently of one another, by CN, CH3 or F, 1-cyclohexene-1,4-diyl, in which one H atom may be replaced by F, pyridine-2,5-diyl, unsubstituted or monosubstituted by F, thiophene-2,5-diyl, unsubstituted or monosubstituted by F; furan-2,5-diyl or naphthalene-2,6-diyl, unsubstituted or monosubstituted or disubstituted by CN or F;
M1 is undirected and is xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94SC(xe2x95x90O)xe2x80x94, xe2x80x94SCH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94OC(xe2x95x90O)CH2CH2xe2x80x94, xe2x80x94OCH2CH2CH2xe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94(CH2)4xe2x80x94 or a single bond;
a is 0 or 1.
The symbols and indices in the formula (I) preferably have the following meanings:
R1 and R2 are preferably identical or different and are each hydrogen or a straight-chain or branched C1-C18-alkyl or C2-C18-alkenyl radical (with or without asymmetric carbon atoms), where
a) one or two nonterminal CH2 groups may be replaced, independently of one another, by xe2x80x94Oxe2x80x94 or xe2x80x94C(xe2x95x90O)xe2x80x94, with the proviso that heteroatoms cannot be linked directly to one another, and/or
b) one or more H atoms may be replaced by F,
c) in the case of an alkyl or alkenyl radical containing asymmetric carbon atoms, the asymmetric carbon atoms have xe2x80x94CH3, xe2x80x94CF3 and or xe2x80x94F as substituents,
with the proviso that only one of R1 and R2 can be hydrogen.
A1 and A2 are preferably undirected and identical or different and are each phenylene-1,4-diyl, unsubstituted or monosubstituted or disubstituted by F, cyclohexane-1,4-diyl, 1-cyclohexene-1,4-diyl, pyridine-2,5-diyl, unsubstituted or monosubstituted by F, or thiophene-2,5-diyl.
M1 is preferably undirected and is xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94OCH2xe2x80x94 or a single bond.
a is preferably 0.
Particular preference is given to the following compounds of the formulae (I-1) to (I-12). 
in which:
R3 is hydrogen or a straight-chain or branched C1-C15-alkyl or C2-C15-alkenyl or C1-C15-alkyloxy radical (with or without asymmetric carbon atoms),
R4 is hydrogen or a straight-chain or branched C1-C15-alkyl or C2-C15-alkenyl radical (with or without asymmetric carbon atoms), in which one nonterminal CH2 group may also be replaced by xe2x80x94Oxe2x80x94 and in which one or more H atoms may be replaced by F,
with the proviso that only one of R3 and R4 can be hydrogen.
Particular preference is given to compounds of the formula (I), in particular (I-1) to (I-12), in which R3 is a straight-chain C1-C15-alkyl or C2-C15-alkenyl radical and R4 is a straight-chain alkyl or alkyloxy radical having 1 to 15 carbon atoms.
xe2x80x9cUndirectedxe2x80x9d means that incorporation of the group in the form of its mirror image is possible.
xe2x80x9cTerminalxe2x80x9d means, for example in R1, the CH2 groups connected to H.
Of the compounds of the formula (I) which are to be used as optically active components (dopants), preference is given to those in which the alkyl group contains the asymmetric carbon atoms in the form of at least one of the following groups:
a) xe2x80x94C*H(CH3)CmH2m+1, where m has a value of from 2 to 8
b) xe2x80x94OC*H(CH3)CmH2m+1, where m has a value of from 2 to 8
c) xe2x80x94OC*H(CH3)CO2CmH2m+1, where m has a value of from 1 to 10
d) xe2x80x94OC(xe2x95x90O)C*H(CH3)OCmH2m+1, where m has a value of from 1 to 10
e) xe2x80x94OC(xe2x95x90O)C*H(F)CmH2m+1, where m has a value of from 1 to 10
f) xe2x80x94OCH2C*H(F)CmH2m+1, where m has a value of from 1 to 10
g) xe2x80x94OCH2C*H(F)C*H(F)CmH2m+1, where m has a value of from 1 to 10
h) oxirane-2,3-diyl
in which C* denotes the asymmetric carbon atom.
The compounds according to the invention are prepared by methods known per se from the literature, as described in standard works on organic synthesis, for example Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart.
However, it may prove necessary to vary or modify the literature methods for the requirements of mesogenic units, since, for example, functional derivatives having long ( greater than C6) alkyl chains tend to be less reactive than, for example, the methyl or ethyl analogues.